This invention relates to a circuit for using a piezoelectric crystal, or any other type of electromechanical resonator capable of vibrating in several different modes, as the stable frequency reference element in an oscillator circuit. More particularly, the output of one or more voltage controlled oscillators is used to drive the crystal and the output frequencies passed by the crystal are supplied to a corresponding number of phase detectors. Each phase detector is also supplied with the output of a voltage controlled oscillator and generates a DC voltage which is proportional to the phase difference between the output voltage of the voltage controlled oscillator and the output of the crystal, the proportional voltage being used to bring the frequency of the voltage controlled oscillator onto the crystal frequency.
It is a familiar fact that a piezo-electric crystal can be used in different resonant modes as the frequency determining element in an oscillator circuit. Also, a single crystal vibrating in several modes can be used to generate several frequencies simultaneously.
The typical classical, single frequency, crystal controlled oscillator may have an amplifier feeding its output back to its input through a filter network. The amplifier must have a broad bandwidth in order to have good phase stability and is provided with automatic gain control for regulating the amplitude of oscillation. The automatic gain control circuit, in combination with the network, is used to shape the gain and phase in such a way that oscillation occurs at only one frequency. A major source of error of the oscillator frequency is lack of phase stability in the amplifier and the network; any change in phase results in movement of the crystal off of resonance to compensate for it.
When the oscillator circuit just described is used to generate multiple frequencies, individual amplifiers and compensating networks must be provided in parallel loops between the crystal output and input in order to control the gain for each frequency. Since the Q of the crystal is different for each resonant frequency, the required gain in each path is different. There are difficulties with the implementation of this type of circuit. The various filters required for each frequency must be relatively sharp in order to separate the frequencies; such filters tend to produce large phase changes in response to minor variations in component parameters. Any phase change produced by the filters will result in unacceptably large changes in crystal operating point to compensate for the change. This circuit is also deficient in that frequency instability can result from interaction between the controls, particularly when the frequencies are not sufficiently separated. Thus, leakage of a signal at one frequency may occur through the filter for another frequency, and vice versa, and, since the signals will be considerably shifted in phase, each affected crystal mode will be required to move further off resonance to compensate.